This invention relates to an apparatus and method for forming drainage holes in corrugated plastic drainage pipe. Many drainage pipes are produced from plastic materials, either by extrusion or blow molding, with the pipe being cut into finite lengths or carried as an endless pipe on a large drum or sizing in the field. To obtain high rigidity and maximum utilization of the material, such drainage pipes are commonly corrugated with the corrugations extending circumferentially around the pipe. Preferably, drainage holes are positioned in the wall of the inner corrugation so that, when the pipe is ultimately buried in the earth, the drainage holes are not packed as tightly as would be the case if they were positioned in the outer corrugation. This facilitates drainage from the pipe, but, for obvious reasons, complicates the method for forming the perforations.
It is also desirable that the perforations be relatively large in size and that they have no protruberance or obstructions to the flow of liquid in the pipe. For example, a hole formed by a drill or cold punch will leave a burr or other extending protruberance within the pipe which will snag and entrap small roots or other fibrous matter entrained in the liquid being carried. This will eventually cause a buildup of solids at that point, thereby closing the hole and ultimately reducing the effective diameter of the pipe itself.
One method of perforating such drainage pipes is disclosed in U.S. Pat. No. 2,697,264. This method limits itself to perforating only a short length of pipe, since it utilizes a mandril to support the pipe for punching. Other methods of perforation, such as shown in U.S. Pat. Nos. 2,834,983 and 3,892,514 disclose apparatuses for perforating the pipe simultaneously with the manufacture of the pipe itself. The obvious limitation in these methods is that they can make only perforated pipe, thereby requiring a separate machine to produce non-perforated pipes. Finally, recently issued U.S. Pat. No. 4,055,098 shows a pipe perforating apparatus which uses a cold punch and provides means for supporting the pipe in the punching area to prevent deformation thereof. While the aforementioned patents describe various methods and apparatuses for perforating pipe in the manner described which have been used commercially, each has certain limitations which limit their effective use, such as cost of installation, inability to provide clean perforations free of burrs or protruberances, etc.